Uranocene, U(C₈H₈)₂, is an

organouranium compound Organouranium chemistry is the science exploring the properties, structure and reactivity of organouranium compounds, which are organometallic compounds containing a carbon to uranium chemical bond. The field is of some importance to the nuclear ...

composed of a uranium atom sandwiched between two

cyclooctatetraenide 1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of ...

rings. It was one of the first organoactinide compounds to be synthesized. It is a green air-sensitive solid that dissolves in organic solvents. Uranocene, a member of the "

actinocenes Actinocenes are a family of organoactinide compounds consisting of metallocenes containing elements from the actinide series. They typically have a sandwich structure with two dianionic cyclooctatetraenyl ligands (COT2-, which is ) bound to an a ...

," a group of

metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...

s incorporating elements from the

actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...

series. It is the most studied bis nnulene-metal system, although it has no known practical applications.

Synthesis, structure and bonding

Uranocene was first described in 1968 by the group of

Andrew Streitwieser Andrew Streitwieser was an American chemist known for his contributions to physical organic chemistry. Streitwieser was born in 1927 in Buffalo, New York and he grew up in New York City. He attended Columbia College and then Columbia Univers ...

, when it was prepared by the reaction of

dipotassium cyclooctatetraenide Dipotassium cyclooctatetraenide, sometimes abbreviated K2COT, is an Organopotassium chemistry, organopotassium compound with the formula K2C8H8. It is a brown solid that is used as a precursor to cyclooctatetraenide complexes, such as uranocene ( ...

and

uranium tetrachloride Uranium tetrachloride is an inorganic compound, a salt of uranium and chlorine, with the formula UCl4. It is a hygroscopic olive-green solid. It was used in the electromagnetic isotope separation (EMIS) process of uranium enrichment. It is one o ...

in THF at 0°C: : Uranocene synthesis

Uranocene is highly reactive toward oxygen, being

pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...

in air but stable to

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

. The x-ray crystal structure of uranocene was first elucidated by the group of

Ken Raymond Kenneth Norman Raymond (born January 7, 1942) is a bioinorganic and coordination chemist. He is Chancellor's Professor of Chemistry at University of California, Berkeley, Professor of the Graduate School, the Director of the Seaborg Center in t ...

. Considering the molecule to be U⁴⁺(C₈H₈²⁻)₂, the η⁸-

groups are planar, as expected for a ring containing 10 π-electrons, and are mutually parallel, forming a

sandwich A sandwich is a food typically consisting of vegetables, sliced cheese or meat, placed on or between slices of bread, or more generally any dish wherein bread serves as a container or wrapper for another food type. The sandwich began as a po ...

containing the

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

atom. In the solid state, the rings are eclipsed, conferring ''D_8h'' symmetry on the molecule. In solution the rings rotate with a low

activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...

. The uranium-cyclooctatetraenyl bonding was shown by

photoelectron spectroscopy Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in th ...

to be primarily due to mixing of uranium 6d orbitals into ligand pi orbitals and therefore donation of electronic charge to the uranium, with a smaller such interaction involving the uranium (''5f'')² orbitals. Electronic theory calculations agree with this result and point out that the weaker interaction of the open-shell ''5f'' orbitals with the ligand orbitals determines , ''M_J'', , the magnitude of the angular momentum quantum number along the 8-fold symmetry axis of the ground state.

Spectroscopic properties

Uranocene is

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, d ...

. Its

magnetic susceptibility In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...

is consistent with values of 3 or 4 for , ''M_J'', , with the accompanying magnetic moment being affected by the spin-orbit coupling. Its

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...

spectrum is consistent with an , ''M_J'', value of 3. Electronic theory calculations from the simplest to the most accurate also give , ''M_J'', values of 3 for the ground state and 2 for the first excited state, corresponding to double-group symmetry designations of ''E_3g'' and ''E_2g'' for these states. The green color of uranocene is due to three strong transitions in its

visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...

. In addition to finding vibrational frequencies, Raman spectra indicate the presence of a low-lying (''E_2g'') excited electronic state. On the basis of calculations, the visible transitions are assigned to transitions primarily of ''5f''-to-''6d'' nature, giving rise to ''E_2u'' and ''E_3u'' states.

Analogous compounds

Analogous compounds of the form M(C₈H₈)₂ exist for M = ( Nd, Tb, Yb, Th, Pa, Np, and Pu). Extensions include the air-stable derivative U(C₈H₄Ph₄)₂ and the cycloheptatrienyl species (C₇H₇)₂sup>−. In contrast,

bis(cyclooctatetraene)iron Bis(cyclooctatetraene)iron is an organoiron compound with the formula Fe(C8H8)2, abbreviated Fe(COT)2. It is an air-sensitive black solid that is soluble in diethyl ether and aromatic solvents. The compound decomposes in solution after a few days ...

has a very different structure, with one each of a η⁶- and η⁴-C₈H₈ ligands.

Synthesis, structure and bonding

Spectroscopic properties

Analogous compounds

References

Further reading